1. Field of the Invention
The present invention relates to erasable optical disk media used as data files or digital audio disks for recording (writing), reproducing (reading) and erasing digital information by a light beam. It also relates to a method of recording/erasing for such erasable optical disk media.
2. Description of the Prior Art
In recent years, optical disks have been applied to various uses such as data files as they have the advantage of a high density, a large capacity, a high access speed, etc.
The optical disks are generally classified into three types: "Read-only" disks in which instructions or data are permanently stored and the stored information is read out nondestructively, and no information can subsequently be recorded or written into this read-only optical disks; "Write-once" disks in which data can be written in addition to the permanently stored information, and the stored information is read out nondestructively; and "Erasable" disks in which information can be written or recorded and erased repeatedly. Among others, a great concern is given to the erasable optical disks to which the present invention pertains.
The erasable disks are classified into several types by processes used. These types include the "Phase change" type in which heat of a laser beam is used to create a phase change in an optical recording medium between the crystalline phase and the amorphous phase, and changes in reflective index in the phase change area are read out; the "Magneto-optic" type in which a recording layer is heated above the Curie temperature by means of a laser beam and while at the same time, a magnetic field is applied to change the magnetic polarization of pits, and changes in plane of polarization of light caused by Kerr effect are read out; the "Photochromic" type which is based on shifts in light absorption wavelength band observed when organic dyes are exposed to a light beam; and the "Double layer" type in which the heat of a laser beam is used to create and erase dome-like bumps. More detailed information is available, for example, from the article "Current Condition and Trend of Optical Disk Recording Materials", M. Kobukata et al., OPTRONICS (1987) No. 12.
The phase change disks have a drawback that pits tend to spread out due to thermal conduction of the optical recording medium when the disks are rotated at a low linear speed. The magneto-optic disks are also disadvantageous for its high material cost and the necessity of a complicated optical system. The photochromic disks have a problem for the deterioration of signals. With the difficulties in view, the present invention is directed to a double layer optical disk wherein information is written or recorded by the formation of dome-like protuberances or bumps.
The double layer optical disks include two types: One is composed of a first layer of metal or synthetic resin and a second layer of shape-memory alloy (hereinafter referred to as "metal-alloy" type); the other is composed of two layers of organic dyes having different absorption bands (hereinafter referred to "double layer organic dye" type). Typical examples of such double layer optical disks are disclosed in Japanese Patent Laid-open Publication Nos. 56-124136 and 60-69846.
Conventional erasable optical disks of the metal-alloy type and the double layer organic dye type will be described below with reference to FIGS. 10 and 11 and FIG. 12 of the accompanying drawings.
FIG. 10 is a cross-section of the conventional metal-alloy type erasable optical disk as it is in the initial or non-recorded state. FIG. 11 is a cross-section similar to FIG. 10, but showing the metal-alloy type in the recorded state. In FIG. 10, 101 is a clear or transparent substrate, 102 is a first layer of metal or synthetic resin, and 103 is a second layer of shape-memory alloy. In FIG. 11, 111 is a pit and 111 denotes a space.
The transparent substrate 101 is made of glass or synthetic resin having a low coefficient of linear thermal expansion and supports thereon the second layer of metal or synthetic resin having a high coefficient of linear thermal expansion. The shape-memory alloy layer 103 is formed over the second layer 102 of metal or synthetic resin.
From the backside of the transparent substrate 101 an intermediate power laser is irradiated on the first layer 102 of high coefficient of thermal expansion and the first layer 102 expands to thereby form a pit 111 and a space 112 as a result of formation of the pit 111. In this instance, the shape-memory alloy layer 103 is heated at a temperature below its melting point and hence undergoes plastic deformation with the result that the shape of the pit 111 is set. Erasure is performed by irradiating a high power laser beam on the shape-memory alloy layer 103 to heat the same to a temperature exceeding the transforming point of the martensitic phase. The second shape-memory allow layer 103 recovers its original shape so that the dome-like bump (pit) is erased.
FIG. 12 is a cross-section of a double layer organic dye type erasable optical disk. The optical disk consists of a substrate 121, a thermal expansion layer 122 having an absorption band at about 830 nm, and a retention layer 123 having an absorption band at about 780 nm. More concretely, the thermal expansion layer 122 is composed of an organic dye having an absorption band at about 830 nm and a viscoelastic binder (a synthetic resin). The thermal expansion layer 122 is formed as a first layer on one surface of the substrate 121 by the spin coat process. The retention layer 123 is composed of an organic dye having an absorption band at about 780 nm and a synthetic resin having a glass-transition temperature higher than that of the first layer 122. The retention layer 123 is formed as a second layer on an outer surface of the first layer 122 by the spin coat process. For recording, a laser beam of 830 nm is irradiated from the backside of the substrate 121 whereupon the first layer 122 having the absorption band of 830 nm is heated mainly. The first layer 122 thus heated undergoes thermal expansion to deform the retention layer 123 plasticly and to form a dome-like bump (or pit) for recording a signal. Erasing is performed by irradiating the second layer 123 with a laser beam having a wavelength of 780 nm. The second layer 123 having the absorption band of 780 nm is heated and becomes soft so that the dome-like bump (pit) is flattened or erased by residual stress stored in the thermal expansion layer 122.
The metal-alloy type erasable optical disk of the foregoing construction has a drawback that the first layer, as it is separated from the transparent substrate 101 in the recording process, is instable in shape and hence the size of the dome-like bumps is not uniform. In the erasing process, the space still exists between the transparent substrate 101 and the first layer 102, leading to an incomplete erasure. After erasing, the temperature of the shape-memory alloy layer 103 goes down while at the same time, the first layer 102 is still hot and expands again. This brings about an accidental recording of digital information. A further drawback involved in the metal-alloy type is that the shape-memory alloy layer 103 has a tendency to change into the amorphous phase when subjected to thermal shock during repeated recording (writing) and erasing operations. The life time of the metal-alloy type is relatively short.
The double layer organic dye type erasable optical disk has a drawback in that due to its structure composed of two synthetic resin layers, the thermal resistibility of this erasable optical disk is low and when used in the environment at a temperature above 70.degree. C., a notable drop of carrier-to-noise (C/N) ratio takes place.